Fast evaluation of density functional exchange-correlation terms using the expansion of the electron density in auxiliary basis sets

The most time-consuming step in molecular calculations using approximate density functional theory is the evaluation of Coulomb and exchange-correlation terms. We investigate the possibility of fast and sufficiently accurate evaluation of both terms using the expansion of molecular electronic density in atom-centered auxiliary basis sets. Such an approach is shown to be about an order of magnitude faster than usual approaches in which only Coulomb terms are treated using the approximated density. Test calculations suggest that auxiliary basis sets of moderate size are sufficient to achieve good accuracy of molecular properties such as geometries and reaction energies.